The Ark of Inquiry seeks to support inquiry-based science education (IBSE) in different countries and school systems across Europe by teachers that may differ in light of their prior experiences with IBSE. Given the differences, the assumption is that teachers need to make adaptations to the approach and materials of the Ark of Inquiry. This study follows 20 primary school teachers from the Netherlands as they apply the Ark of Inquiry approach and materials in their classrooms, and seeks answers to the research questions if, how and why the teachers make adaptations to the approach and materials. The collected data include lesson plans and diaries of the teachers before and during the implementation, and group interviews held with the teachers afterward. The findings show that teachers appreciate and successfully implement the three core elements of the approach (a five-phase model, formative evaluation, and responsible research and innovation). While doing so, teachers frequently adapt materials to their own and their pupils' needs. Examples of adaptations are changing the activity level, adjusting evaluation instruments, and adding creative components to activities. Reasons to make adaptations are both practical (e.g., time constraints and classroom management) and pedagogical (e.g., preferring group work and alignment with age and capacities of pupils). From this study, it is concluded that the fidelity of implementation concerning the approach is high, and at the same time, the materials provide a rich and relevant starting point for further adaptation. The outcomes support the idea that turning teachers into designers by promoting and supporting adaptation strengthens successful local implementation while leaving the principles of the approach intact.